1. Field of the Invention
The present invention relates to frangible, ceramic conveyor rolls designed for use in the heat treatment of glass sheets wherein the rolls are rotated to transport a series of glass sheets through a hot enclosed atmosphere.
Furnaces for tempering or annealing glass conventionally include conveyors which carry glass sheets through an enclosed atmosphere of high temperature during the annealing or tempering of the glass. A so-called roller hearth-type of conveyor utilized to carry glass sheets includes horizontally extending conveyor rolls of ceramic material on which heated glass sheets are rotatably supported. Rotating the conveyor rolls drives the glass sheets by friction over the rolls through an enclosed furnace atmosphere of high temperature.
One ceramic material utilized to form cylindrically shaped rolls for a roller hearth is a sintered fused silica material that is made by grinding solid fused silica into small particles and then sintering these particles to each other at a temperature below the fusing temperature. Such a material is readily formed into an elongated shape required for the rolls of the roller hearth. In addition, fused silica rolls so formed have a relatively small coefficient of thermal expansion. This latter characteristic is extremely desirable due to extreme temperatures to which the rolls in the roller hearth are subjected during use.
One conventional way for rotatably supporting and driving conveyor rolls of fused silica and other ceramic compositions utilizes bearings that support the opposite end portions of the rolls in the roller hearth. A drive member having a toothed drive component such as a gear or a chain sprocket drives the roll at one end. A metallic end cap rotatable with a driven shaft is conventionally used to transmit the drive from the drive gear or chain sprocket to the end of the ceramic roll.
Rolls of ceramic composition are superior to metal rolls in high temperature environments for several reasons. First of all, ceramic rolls have a low coefficient of thermal expansion compared to that of metal rolls so that they are less likely to warp. Ceramic rolls are less likely to oxidize and flake off and develop pock marking in their glass engaging surface. However, ceramic materials are difficult to work and are quite frangible. Furthermore, such rolls composed of ceramic material cannot be connected directly to the drive mechanisms needed to rotate the rolls. The use of metal end caps with ceramic rolls for roller hearths has been developed to a certain extent. However, they have introduced problems of securing the metallic end caps to the ends of the ceramic rolls that they drive in transmitting a torque from the driving mechanism. One method involves an adhesive bond between the end cap and the end of the ceramic roll. Another method involves shrinking a metallic end cap onto the roll end by heat shrinking. Both of these methods have drawbacks. Adhesives presently known require an extended cure period of at least about 24 hours before the ceramic roll is securely fixed to its end cap. Such a delay is too long to accept for mass production operations.
Heat shrinking requires heating the metallic end cap to a very high temperature so that its diameter increases to enable it to be shrunk fit over the end of the ceramic roll. The internal diameter of the end cap and the outer diameter of the ceramic roll must be closely matched so that the end of the roll is small enough to be inserted into the heated end cap but large enough to be securely clamped after cooling. Often it is necessary to machine one or both of the matching parts in order to provide suitable matching. At the inner end of the metallic cap, the ceramic roll is subjected to a clamping pressure due to the heat shrinking of the end cap. Adjacent to this portion of the ceramic roll subjected to the clamping pressure is an adjacent portion free of the clamping pressure. At this location of the boundary between a high clamping pressure and no clamping pressure, ceramic rolls are subject to fracture due to the high stresses involved.
2. Description of the Prior Art
U.S. Pat. No. 3,608,876 to Leaich et al discloses pin and slot connections between ceramic hearth rollers and metal shafts. The fragility of ceramic rolls makes such pin and slot connections impractical.
U.S. Pat. No. 3,867,748 to Miller discloses the use of an adhesive for securing metal end caps to the ends of cylindrical ceramic conveying rollers in a roller hearth. The rolls cannot be used until the adhesive is cured. Since curing is a slow process, such rolls are not practical for rapid installation for high-speed production operations.
U.S. Pat. No. 4,131,420 to Miller utilizes a two-piece coupling that disconnects a ceramic roll including its end cap from a drive train which provides a rotating force to the end cap of the ceramic roll. The end cap in this patented apparatus is attached to the ceramic roll using an adhesive in a manner similar to that of the previous patent.
U.S. Pat. No. 4,140,486 to Nitschke utilizes a metallic spring end cap for rotatably driving a glass conveyor roll of ceramic material using helical spring coils that have a diameter in an undeflected condition slightly less than the diameter of the end of the ceramic roll so that coil deflection increasing the coil diameter allows mounting the coils over the end of the ceramic roll and a subsequent release clamps the coils over the end of the roll. The helical spring coil may be engaged by friction or by tooth sprocket or gear driven rolls to transmit a force thereto.